Using the TDM service

Using the TDM service

Assay service

The assay service is routinely available Monday to Friday, 9am-5pm.  For urgent assay requests for phenytoin, phenobarbitone, carbamazepine, primidone, ethosuximide, topiramate and valproic acid (please contact the Unit in advance), the Unit will report on blood samples arriving in the laboratory between 9am and 5pm within 30 minutes.  The Unit will report on non urgent assay requests by the following day and specialist assay requests within 24 working hours. 

During recent years the Unit has greatly increased its repertoire of drug assays available on a ‘routine’ basis.  Many of these assays were developed for colleagues with a specific clinical or research interest or as part of the Unit’s research programme.  If there is a specific assay that you require but is not listed in the table, please contact the Head of Unit.

On-call service

The Unit does not provide an on-call service.

Requesting

Please use the Antiepileptic Drug Level (Concentration) Request Form (NSE01) (pdf).  The form needs to be completed appropriately and clearly to enable efficient processing of assay request.

Reason for request

In order to aid interpretation of results and to identify additional test requirements, it is helpful if the reason for the assay request is indicated.  This can be indicated by ticking one of the five boxes printed on the request form or by specifying accordingly.  This information is also useful for audit purposes.

Specimens and sampling time

An appropriate specimen is essential for effective monitoring.  This requires that the patient is at steady-state on the present dose of the drug, except when suspected toxicity is being investigated, when waiting to attain steady-state is clearly contraindicated.  Steady-state concentrations (levels) can be expected to be achieved when five half-lives have elapsed, unless loading doses are employed when they are attained more rapidly. 

5 mls of blood collected into a plain glass tube or lithium heparin tube to provide serum or plasma respectively is usually sufficient for most assays or assay groups. 

All tests are available for blood and saliva.  All AED assays are available as total plasma/serum concentrations or as free non-protein-bound concentrations.

Collection of saliva samples

1. The ideal sampling time is just before the ingestion of the next scheduled drug dose.  If this is not possible, sampling should occur at least 2-3 hours after drug ingestion.
2. If possible, stimulate salivary flow by chewing of paraffin wax or a rubber band.
3. Saliva can be removed from the patients’ mouth by use of a suitable syringe.
4. The saliva collected during the first 2 minutes should be discarded.
5. Saliva (1-2 mls) should be collected into a suitable container and stored frozen until dispatch to the laboratory for analysis.  If the saliva sample is to be dispatched on the same day, there is no need to freeze the sample.

Urgent specimens must be indicated on the request form by ticking the ‘Urgent’ box or clearly writing ‘URGENT’.  The requesting doctor should clearly indicate on the request form a bleep number or a contact telephone number.

It is the doctor’s duty to ensure that laboratory staff are informed of known and suspect high risk specimens.

Ideal sampling time

This should be adhered to whenever possible.  Drug concentrations (levels) measured at these times provide more meaningful information.  Samples not taken at the ‘ideal time’ can be interpreted if the exact time of drug dose and actual sample time are known.  Please clearly indicate this information on all request forms. (See the table of AEDs (pdf).)

Reference range

This is the range associated with optimal efficacy and minimal toxicity for the majority of patients.  It is important to recognise that some patients will require concentrations (levels) outside of the quoted reference range for optimal clinical response. (See the table of AEDs (pdf).)

Time to steady-state

This is the earliest time a drug concentration (level) should be measured either following initiation of therapy or a change of dosage (unless therapeutic failure or toxicity is suspected).  During suspected toxicity, sampling should be undertaken at a time when adverse events are presenting. (See the table of AEDs (pdf).)

Reports

Reports are printed, upon completion and validation of drug assay, throughout the day.  Assay requests indicated as ‘Urgent’ will be telephoned to the requesting doctor provided a telephone number or bleep number is clearly indicated on the request form.

Target turnaround times

Carbamazepine, Ethosuximide, Phenobarbitone, Phenytoin, Primidone, Topiramate and Valproic acid: within 2 working hours.

Carbamazepine-epoxide, Clobazam, Clonazepam, Diazepam, Felbamate, Gabapentin, Lamotrigine, Levetiracetam, Oxcarbazepine, Pregabalin, Tiagabine, Vigabatrin and Zonisamide: within 24 working hours.

AEDs that are routinely montiored

Sample information

In order for the sample to be processed efficiently and for the results to be interpretable, it is important that the following information is provided:

1. Patient name, sex and date of birth
2. Antiepileptic drugs for which analysis is requested, including prescribed dose(s)
3. Time of saliva sampling
4. Time of drug ingestion

If it would be helpful, please use the Unit’s dedicated request form.

Dispatch of samples

Samples (unfrozen) should be dispatched by first class post, using packaging currently recommended for transporting pathological samples by post and forwarded to: Dr Neville Ratnaraj, Therapeutic Drug Monitoring Unit (TDM), Epilepsy Society, Chalfont St Peter, Chesham Lane, Buckinghamshire, SL9 ORJ.